The present invention relates to single air conduit refrigerated display cases having an ambient air defrost system. Of primary concern are display cases having access openings in their front walls. Both within the specification and the claims of the present application, all references to refrigeration apparatus or refrigeration operations are intended to include cooling both at a temperature below 32.degree. F., such as associated with frozen food display cases, and in excess of 32.degree. F., such as typically associated with dairy food and fresh meat display cases.
Refrigerated display cases having either front and top access openings have been used for many years. Such open front cases are conventionally utilized for displaying dairy and meat products.
In the operation of all types of refrigerated display cases, it is desirable to include a system capable of automatically defrosting the display case. The defrost cycle can be actuated either at set periodic times or when the frost buildup within the system has reached a certain predetermined level. Such systems are typically thermostatically controlled so as to switch from a refrigeration cycle to a defrost cycle of operation. By this manner of operation, it is possible to avoid any significant frost buildup with the display case.
Typically within the prior art, there have been three different approaches employed for defrosting refrigerated display cases. The first approach involves the use of electric resistance heaters that are arranged adjacent to the refrigeration coils of the refrigeration mechanism. During a defrost cycle, these heaters supply heat in an effort to melt the frost buildup on the coils and to supply warmer air for circulation with the case. This particular technique is relatively simple both in its construction and operation. However, since the electrical heaters are high voltage heaters that utilize significant electricity during operation, with the rapidly increasing cost of electricity it has become extremely uneconomical to employ such systems. Furthermore, the warm air circulated in the case can raise the temperature of the case too high. Thus, attempts have been made to find other alternatives to such a system.
A second type of system circulates hot compressed gaseous refrigerant through the refrigeration coils during the defrost cycle. During the defrost cycle, a valve control mechanism shuts off the supply of refrigerant to the refrigeration coils and alternatively feeds superheated compressed gaseous refrigerant through the coils. This hot gas serves to melt any frost buildup that has accumulated on the refrigeration coils but simultaneously provides heat within the air conduit which can be circulated through the display case, which again is disadvantageous. While this type of system does not suffer from the high cost of operation of the electrical heater defrost system, the heated gas system involves a relatively high construction cost. Due to the requirement that the system be able to selectively switch between the supply of heated gas and refrigerant to the refrigeration coils, a complicated flow system is required which increases the number of complex parts capable of breaking down and necessitating costly repairs.
The third type of system employed for defrosting display cases relies upon ambient air. It is this general category with which the invention of the present application is concerned. One type of system that employs ambient air during the defrost cycle is exemplified by those embodiments illustrated in U.S. Pat. Nos. 3,403,525; 3,850,003; and 3,937,033; all to Beckwith et al. Each of these systems uses fans separate from the main air circulating fans. These extra fans are turned on during the defrost cycle for pulling ambient air from outside of the display case into the air conduits. A second type of system is illustrated in U.S. Pat. No. 3,082,612 to Beckwith, which system draws ambient air into the main circulation path through ports located in the lower front panel of the refrigerated display case. Such ports are normally closed during the refrigeration cycle and are opened during the defrosting cycle. The Bechwith et al '003 patent indicates that the concepts described in U.S. Pat. Nos. 3,082,612 and 3,403,525 did not prove to be practical and hence were not commerically feasible.
A third type of ambient air defrosting system is shown in U.S. Pat. No. 4,144,720 to Subera et al, which is assigned to the same assignee as the present application. In the foregoing patent application, an open front refrigerated display case having primary and secondary air conduits is disclosed. In this system, reversible fans are employed for reversing the direction of flow of air within the conduits and simultaneously drawing in air from outside of the display case.
Another system employing reversible fans for ambient air defrost is shown in U.S. Pat. No. 4,026,121. This patent, however, refers to short-circulating the air flow between the primary and secondary air bands for the purpose of supplying warmer air to the primary band. No means for causing the defrost air to flow outwardly away from the display case is shown.
It has been recognized that an ambient air defrost operation can be incorporated into an open top refrigerated display case as disclosed in U.S. Pat. No. 4,120,174 to Johnston. The Johnston patent illustrates an open top case having a single air conduit extending around the case. During the refrigeration cycle, the air flows in a first direction and during the defrost cycle the direction of the air flow is reversed with ambient air being drawn into the conduit. The quantity of air flow during the defrost cycle is greater than during refrigeration. The defrost air, after passing through the conduit, is expelled in a direction up and over the refrigerated case. It has been found during the development of the present invention, that with a defrost air flow pattern such as disclosed in the Johnston patent a significant portion of the expelled air will fall back towards the access opening in the refrigerated case and reenter the air conduit at the other side of the access opening.
Similar to the open top display case of the above-identified patent to Johnston, there also are open front single air conduit cases which employ the same air defrost techniques as disclosed by the Johnston patent. During the defrost operation of such air defrost cases, the volume of the reverse air flow is relatively high and significantly greater than the air flow during the refrigeration cycle. Such a greater air flow is utilized so as to minimize the time needed for the defrost operation. As shown in FIG. 1, which illustrates this type of prior art display case, the higher air volume flow causes the warmer defrost air to reenter the display case which potentially can damage the products in the case. In addition the greater air flow causing the cold air to flow upwardly can result in hitting the customer in the face with such cold air.
Japanese Pat. No. 32,154 to Takizawa shows a refrigerated display cabinet in which a particular evaporator coil box suspension arrangement is shown in FIGS. 2-4. The English language abstract does not indicate an air defrost cycle and no air flow control means for use during air defrost to eject an air band away from the cabinet is set forth. The suspension arrangement may even prevent the use of air defrost.